Sherman DD: The Swimming Tank of D-Day
The Sherman DD (Duplex Drive) was a standard M4 Sherman tank fitted with a collapsible canvas flotation screen and twin propellers. When the screen was raised, it displaced enough water to float the 33-ton tank, barely. The tank would launch from a landing craft offshore, swim toward the beach under propeller power at about 4 knots, then drop the canvas screen upon reaching land and fight as a normal tank. The concept was designed to give infantry immediate armored support during amphibious landings.
On D-Day, June 6, 1944, the DD tanks faced their ultimate test, with mixed results. At Omaha Beach, rough seas swamped many DDs launched 5,000 meters offshore; 27 of 29 tanks from one battalion sank, drowning most of their crews in one of D-Day's worst tragedies. At other beaches with calmer water and closer launch points, the DDs performed well, reaching shore ahead of the infantry and providing crucial fire support. The DD tank concept was sound in moderate seas but fatally fragile in rough conditions, a 6-inch canvas freeboard separating a functioning tank from a 33-ton anchor on the ocean floor.
